The invention relates to a heating device for supplying heat to a heat user, comprising a closed space within which one or more closed reservoirs are arranged which in operation contain a heat-accumulating material which can be heated by means of at least one primary heat source. The said heat-accumulating material is adapted to act as a secondary heat source for exchanging heat, via one or more heat-transmitting reservoir walls, with a heat transport medium which is present in the closed space and which transports heat from the reservoirs to the heat user by an evaporation/condensation cycle. Heating devices of the kind set forth are known from the co-pending U.S. Ser. No. 262,665 and U.S. Pat. No. 3,817,322.
The heat-accumulating material may be a material which remains in the solid state in any operating condition (for example, Al.sub.2 O.sub.3.sup. . BeO; TiO, MgO.sub.2 ; SiO.sub.2), so that only sensible heat is stored, or a material in which the storage of heat is achieved mainly by utilizing the transition from the solid to the liquid phase (melting heat). Examples of the latter materials are: LiF; CaF.sub.2 ; SrF.sub.2 ; NaCl and other metal salts or mixtures thereof.
If a plurality of reservoirs are present for the heat-accumulating material, they may be arranged to be independent of each other or be connected in series with or parallel to each other by way of one or more common filling and/or discharge ducts.
Because the heating device is operated over a wide temperature range (from room temperature to temperatures beyond 1500.degree. C.), the reservoirs cannot be completely filled, because the fact that the volume of the heat-accumulating material increases as the temperature increases should be taken into account. The volume increase of melting heat-accumulating metal salts generally amounts to 20% to 30%, going from room temperature to operating temperature.
In practice, this fact is usually taken into account by choosing a suitable filling degree for the reservoir, even though other solutions are also feasible, for example, the use of an overflow reservoir for liquid metal salt.
The following problem occurs in the known heating devices. In order to ensure that the evaporation/condensation process of the heat transport medium in the closed space can properly take place, this space is normally evacuated. A substantial vacuum then prevails in the closed space at room temperature. As the operating temperature of the device increases, the vapour pressure of the heat transport medium in the closed space strongly increases. This means that the reservoir walls on the outside are subject to strongly varying pressures, the largest pressure occurring at the highest operating temperature.
Inside the partly filled reservoirs, however, usually a very low pressure prevails, because the reservoirs are usually also evacuated after provision of the heat-accumulating material and before closing, notably to prevent oxidation of the heat-accumulating material by oxygen present in the air. As the temperature of the heat-accumulating material increases, the pressure inside the reservoirs remains constantly low. This is not only applicable to solid heat-accumulating materials, but also to materials which change over to the liquid phase when heated. This is because the vapour pressures of the commonly used heat-accumulating metal salts in liquid form are very low at the temperature levels occurring (below 1 Torr (= 1 mm mercury pressure)).
As a result, the reservoir walls are subjected to strongly varying and large mechanical stresses, because of the variable pressure difference between the variable heat transport medium pressure in the closed space on the one side, and the substantially constant, very low pressure in the reservoir on the other side. The reservoir walls must thus be thick, since they have to be capable of withstanding the largest pressure differences occurring at the highest operating temperatures. Thick reservoir walls, however, have a high thermal resistance and lead to a heavy construction of the device.
The present invention has for its object to provide an improved heating device in which the described drawbacks have been eliminated in a simple manner.